Monthly Archives: March 2012

Again and again, we have learned that there are several occasions where a single, yet well-executed, digital image may not capture the expected results. Most commonly we see High Dynamic Range images from a series of bracketed frames
providing greater tone values of a scene than possible with a single exposure.

Many pictorial images have a very wide Depth of Field range that somewhat limits the capability to capture front-to-back sharpness in a traditional digital image regardless of the aperture selected. We have also learned this DoF is dependent primarily on Aperture, then on Subject Distance, and lens Focal Length. As we get closer to the image in macro photography, the total measured distance of the DoF gets smaller. Therefore, we need to improve upon this fault.

Like in HDR, with the Extreme Depth of Field process we rely upon the blending of several images into one with greatly extended DoF. Each image is focused at a different distance from the lens. When blended in computer software, part of the resultant image uses near focus detail, part uses mid-focus detail and another uses far focus detail, and so on. Often, as many as 10 or more sequential images are “stacked” and blended into one.

This blending process can include the use of multiple layers in Photoshop or a free software called CombineZ. However, the most powerful software and today’s industry standard is Helicon Focus. The current version is 5.2.16 and its cost ranges from $30 to $200 depending on length of subscription service and number of computers licensed.

In this image of the head of a bee 53 individual images with a different point of focus from the antennae to the rear of the head were made. Each image was spaced 0.005 inch from one another from the front to the back. These multiple images spanned the overall distance of o.265 inch.

The screen shot above is from Helicon Focus. It illustrates the blending of these images into the resultant single Extreme Depth of Field image of the bee. The two images below demonstrate the difference in focus from one of the more front images to one of the most rear images. As illustrated, the DoF of any of the images is nil at this magnification.

A most helpful tool for capturing the many individual images required is a focus slider, a tripod mounted, rail-like device that facilitates the movement of the camera backward and forward at very small increments, thus changing the point of focus. (Moving the camera for focus is done as opposed to rotating the lens barrel normally which would change magnification at this scale.)

In my lab, the camera remains stationary and solidly mounted on a heavy tripod. I use a micrometer stage to move the specimen which provides infinite, yet measured travel over a distance of just more than one inch in measured intervals of 0.001 inch. This process proves most ideal when dealing with insects and other tiny specimens. This equipment set-up with a variety of macro lenses is capable of producing images up to 40X magnification. (See image below.)

As in other high-magnification applications, any movement of the camera or subject is most detrimental to image quality. Therefore, solid mounting is paramount. Bolt everything down on solid, vibration-free surfaces. The heavier, the better. In addition, use an electronic cable release and mirror lock-up to minimize motion at every possible point.

The use of electronic flash with its inherent high speed flash duration is most helpful. This is especially true as at great bellows extension, a lot of light loss is encountered. Flash provides these high light levels required for appropriate exposures. As a side benefit, the use of high shutter speed synchronization will also help reduce image deterioration due to internal camera vibrations. As a rule, I generally use 1/250 to 1/500 second shutter speeds at ISO 100 with flash. With these settings I can use the “sweet spot” aperture of F8.0 for highest resolution.

Extreme Depth of Field techniques are exceptionally useful in macro, but the technique is not limited to macro images alone. Remember, DoF is affected
by aperture, lens-to-subject distance, and lens focal length. So you are able to use EDoF whenever given aperture and selected focal length precludes deep DoF.